The present invention generally relates to printed wiring assemblies (PWAs), and more particularly, is concerned with printed wiring boards (PWBs) of the type having multi-leaded surface mounted devices (SMDs) thereon which are capable of withstanding many temperature cycles.
In today's aviation industry, it is common for a single aircraft to be subjected to several extreme thermal conditions in a relatively short time interval. It is not uncommon for an aircraft to be flying at an altitude of 40,000 feet with an outside temperature of less than -40.degree. F., while only moments earlier it was waiting for a take-off clearance from a hot, humid airport runway. With the current aspirations for trans-atmospheric aircraft, these extreme vicissitudes in the ambient temperature will continue to confront avionics engineers with perplexing problems of increasing difficulty and importance.
One particular problem that is exacerbated by these temperature oscillations is the frequent failure of solder connections between the leads of SMDs and the corresponding pads of PWBs. With such PWAs, temperature swings cause the solder joint between the SMD and the PWB to be subjected to a series of stresses. Typically, the PWBs are of a glass/epoxy laminate or other non-conductive material which has a different coefficient of thermal expansion from the SMDs, which are normally fabricated from ceramic materials. This difference in expansion coefficients results in differing degrees of expansion to occur and the intermediate solder joint to be stressed. This problem is increasingly prevalent in PWAs having large multi-leaded SMDs thereon. Larger SMDs typically have many leads which are very small in comparison to the overall size of the SMD. Furthermore, the lead sizes are not typically changed when the overall size of the SMD is increased; therefore, the ratio of the largest linear dimension of the SMD, which is typically directly related to the stress intensity upon the joint, compared with the lead size, increases whenever the overall size of the SMD increases. Ultimately, the series of differential expansion and contraction places sufficient cumulative stress on the intermediate solder joint to cause both mechanical and electrical failure to occur in large SMDs.
Several alternative connection methods have been used in attempts to extend the number of cycle before failure in temperature cycling. One method has been tried where the SMDs are attached to a PWB or surface similar to conventional PWBs, but in addition, having a heavy metal plate attached to its underside with the SMDs being mounted on the top side. This metal plate is chosen to have a thermal coefficient of expansion which is almost equal to that of the SMDs. This "brute force" approach actually limits the differential expansion that can occur, because the wiring board is physically bound to, and restricted from excess expansion by, the underlying metal plate.
Another method to reduce the thermal cycling problems which has received much attention recently, is to make the connection between the SMDs and the PWBs more flexible, so that the differential expansion can be tolerated. This striving for a move flexible connector has produced several alternative connection techniques. One avenue of though has been to provide a new solder alloy that is much more flexible. This "superflex" solder would permit a differential expansion by having the solder stretch between the SMDs and the PWB. Despite the exhaustive and expensive research performed around the world, a sufficiently flexible solder alloy has not been produced. A second avenue of thought along the lines of increased connection flexibility has been pursued, and currently the method used is to elevate the devices above the PWB with the aid of conductive spring-like connectors, columns or solder, or elongated leads. The extra flexibility is the result of the increased length of the connection.
While these techniques, or variations of them, have been used for reducing solder joint failures in PWAs caused by temperaure cycling, they do have numerous and serious drawbacks. One major problem with the metal plate approach is that the cost and weight of such a board is much more than the typical glass/epoxy PWB. The flexible lead approach also has some serious drawbacks. Problems involving vibrations of the mounted devices occur more frequently when they are elevated. Furthermore, when the devices are elevated the PWB is not as effective as a heatsink. Moreover, the cost of such a design is considerably higher than a typical PWA with SMDs.
Consequently, a need exists for improvement in PWAs having SMDs thereon, and of the type that are subjected to temperature cycling which will result in greater reliability and reduction in cost and weight.